1. Field of the Invention
The present invention relates to liquid crystal devices, and in particular, to liquid crystal devices having alignment layers which also serve to block the DC component of a signal applied thereto and to controls for supplying signals to such liquid crystal devices.
2. Description of Related Art
Light shutters are devices which can be controlled to selectively block or permit the transmission of light therethrough. Light shutters have numerous applications. For example, a single light shutter can be used in systems which transmit data optically to permit or prevent the transmission of optical signals therethrough much like an electrical switching device is used in systems which transmit data electrically. A linear array or a matrix of light shutters can be arranged between a light source and a photosensitive material such as, for example, a photoconductive drum or a belt, in an image producing machine such as a copier, printer, or facsimile machine. As the photosensitive material moves past the array or matrix of light shutters, the light shutters are selectively actuated to block or permit the transmission of light from the light source to the photosensitive material to form a latent image on the photosensitive material. This latent image is, for example, toner developed and then transferred to a sheet of paper or other material to form a permanent image on the sheet.
A matrix of light shutters is also typically used to form a display or display screen wherein the light shutters are selectively actuated to form images on the display screen by controlling the transmission of light through portions of the display screen or by controlling the reflection of light by a surface located behind the display screen. Other uses of light shutters are known and possible and are intended to be covered by the present invention. For example, it is known to use liquid crystal display devices as light shutters in copiers, printers, or the like. See, for example, Xerox Corp. U.S. Pat. Nos. 4,506,956, 4,527,864, and 4,475,806.
Liquid crystals are commonly used to form light shutters. Liquid crystals are well known and, generally, are made from materials which exhibit more than one refractive index depending upon their orientation, and whose orientation can be changed by the application of an electric potential.
In a typical transmissive type liquid crystal device, a thin layer of liquid crystal material is sandwiched between parallel, transparent glass substrates bearing transparent, patterned electrodes on their inner confronting surfaces. At least one polarizer is located on the outer surface of one of the glass substrates and a light source spaced from the device directs light therethrough. By selectively supplying an electric field across the layer of liquid crystal material by means of selectively applying, for example, an AC voltage to the electrodes (for nematic-type liquid crystals), the transmissivity of the light through the liquid crystal device may be changed for passing or blocking light in accordance with the electrodes addressed by the voltage.
Liquid crystal materials are organic substances made up of rod-like molecules that are typically about 10 .ANG. long and several .ANG. thick. Within certain temperature ranges, these materials exhibit optical properties of an ordered crystal but have flow properties of liquid.
In the nematic-type of liquid crystal material, which is used with one aspect of the present invention, the center of gravity of the molecules is unordered and random, as in the case of isotropic liquids, but the molecules tend to align themselves with their axis parallel.
Since the individual liquid crystal molecules have an elongated shape and dipoles (both permanent and induced) which are direction dependent, films of these materials exhibit anisotropy in their dielectric constant and refractive index. Materials that exhibit a positive dielectric anisotropy have molecules that tend to align themselves parallel to an applied electric field, while the molecules of materials that exhibit a negative dielectric anisotropy tend to align themselves perpendicular to the field. Because of their optical anisotropy, a change in the orientation of the liquid crystal molecules by an electric field can cause a change in optical transmission when used in conjunction with light polarizing sheets.
By suitable treatment of the inner glass substrates, nematic liquid crystal materials which have a positive dielectric anisotropy are caused to align in a particular direction parallel to the glass substrate surfaces. In one method, the glass substrates may be coated with a thin organic film and conditioned by, for example, rubbing with a lint-free cotton twill cloth in a unidirectional manner. Fine grooves about 50 .ANG. wide are formed causing the liquid crystal molecules to lie substantially parallel to these furrows since this results in a low energy state. Such a conditioned film is generally called an alignment layer or film.
During fabrication the two glass plates are oriented with their alignment directions parallel to each other. A typical transmissive, liquid crystal device that uses a nematic liquid crystal material with a positive dielectric anisotropy comprises two parallel, transparent glass substrates having one or more transparent electrodes (known as pixel electrodes) on the inner surface of one of the glass substrates with at least one electrode on the inner surface of the second glass substrate (known as a backplane electrode) that is opposed to the pixel electrodes on the inner surface of the first glass substrate. A transparent alignment layer covers the pixel and backplane electrodes so that the molecules of the liquid crystal material placed between the glass substrates are parallel to the glass substrate surfaces while they are in their stable relaxed state (with no voltage, or a voltage below a threshold voltage applied). When a voltage above a threshold voltage is applied to the electrodes, the molecules orient themselves perpendicular to the glass substrates and parallel to the direction of the electric field. A polarizer placed on the outside surface of one of the glass substrates, allows the light vector of one direction to pass therethrough but blocks all other light vectors.
When the liquid crystal molecules are lying parallel to the glass substrates in their relaxed state, due to their birefringence, they convert the linearly polarized light passed by one polarizer into elliptically polarized light. Once a voltage is applied to the electrodes, the molecules rotate by 90.degree. to align themselves parallel to the electric field and perpendicular to the glass substrates. This is the electrically driven stable state (or ON state), and in this arrangement, it does not affect the state of polarization of light that travels in a direction essentially perpendicular to the glass substrates. If two polarizers are used on either side of the device, their transmission axes can be either parallel or crossed depending on whether it is desirable that the driven state be clear or dark respectively.
Typically, each pixel electrode is addressed by a thin film transistor (TFT) which acts as a switching device to permit a voltage potential to be applied to each pixel electrode. See, for example, U.S. Pat. No. 4,783,146 to Stephany et al (Xerox Corporation), the disclosure of which is herein incorporated by reference.
It is also known to use ferroelectric materials to form liquid crystals. See U.S. Pat. No. 4,367,924 to Clark and Lagerwall, the disclosure of which is herein incorporated by reference. The ferroelectric liquid crystal in a suitably prepared device has bistability, i.e., has two stable states comprising a first optically stable state (first orientation state) and a second optically stable state (second orientation state), with respect to an electric field applied thereto. Accordingly, the liquid crystal is oriented to the first optically stable state in response to one electric field vector and to the second optically stable state in response to a reversed electric field vector. Further, this type of liquid crystal very quickly assumes either one of the above-mentioned two stable states in response to the direction of an electric field applied thereto and retains such state in the absence of an electric field.
Thus, ferroelectric liquid crystals are polarity sensitive. In any device in which they are used, their response depends upon the sign of the applied voltage as well as upon its magnitude.
While separate pixel electrodes can be supplied for each liquid crystal shutter in an array or matrix of shutters along with corresponding TFT's for each pixel electrode for ferroelectric liquid crystal devices (as with nematic-type devices mentioned above), it is also possible to use an address scheme wherein a first array of parallel electrodes is laid over a second array of parallel electrodes, the arrays being perpendicular to each other, whereby intersections of the electrodes of each array defines each liquid crystal pixel. See, for example, U.S. Pat. No. 4,367,924 to Lagerwall et al, FIG. 2. In this arrangement, the first array of parallel electrodes are located on one glass substrate and the second array of parallel electrodes which are perpendicular to the electrodes in the first array are located on a second substrate, with the ferroelectric material sandwiched therebetween. The electrodes are selectively supplied with voltages so that selected liquid crystal pixels are turned "ON" or "OFF".
An advantage of the electrode layout using two perpendicular arrays of parallel electrodes over the TFT layout is that TFT's may have a low yield and are much more costly to manufacture. That is, TFT's are more difficult to manufacture and, should one TFT in a matrix of TFT's be defective, the entire matrix may need to be replaced, further increasing their costs. Advantages of TFT's are that they have a precise threshold voltage over which they will be turned "ON" and under which they will be switched "OFF". Additionally, when using two perpendicular arrays of electrodes, pixels which are not desired to be switched "ON" can be slightly actuated due to the small voltage applied thereto when other pixels sharing one of its electrodes are switched "ON". This slight switching "ON" reduces the contrast achievable with these devices.
Conventionally, nematic type TFT addressed liquid crystal displays such as shown in the above-incorporated U.S. Pat. No. 4,783,146, were addressed with three voltage level drivers on the data or X-axis, while applying two-voltage level drivers on the strobe or Y-axis. For example, FIG. 1 shows an array of pixel electrodes 100a-100c, each connected to a respective TFT 102a-102c. Each TFT acts as a switching means and is switched ON or OFF based upon the voltage applied thereto from controller 110 through strobe lines Y.sub.1 -Y.sub.4. Each strobe line Y.sub.1 -Y.sub.4 is attached to a gate of a group of TFT's. For example, in FIG. 1, strobe line Y.sub.1 is attached to four TFT's so as to control the state (ON or OFF) of TFT's 102a-102d and, consequently control electrodes 100a-100d. The actual voltage applied to each pixel electrode is supplied by three-level driver 114 through data lines X.sub.1 -X.sub.4. Each data line X.sub.1 -X.sub.4 addresses a single TFT in each group of four TFT's. Thus, each pixel electrode can be selectively addressed by applying an appropriate voltage thereto through data lines X.sub.1 -X.sub.4 when the group of TFT's associated with the pixel electrode is switched ON by the appropriate strobe line. This address scheme is generally well known as shown in the above-incorporated U.S. Pat. No. 4,783,146.
FIG. 2A illustrates the Y-axis waveform (which has two levels), wherein a pulse which switches the TFT's attached to strobe line Y.sub.1 ON is applied to strobe line Y.sub.1 at the beginning of each frame. A frame is a time period during which all of the pixels in the device (image bar, display, etc.) are supplied with an information signal which switches each pixel ON or OFF once (much like a frame of a movie film). Accordingly, within one frame time period, each strobe line Y.sub.1 -Y.sub.4 will receive an ON pulse, usually with each strobe line receiving an ON signal one at a time. FIG. 2B shows the three voltage levels applied by driver 114 to the data lines. In particular, a pulse for applying a voltage above a threshold voltage to data line X.sub.1 for electrode 100a and TFT 102a in three consecutive frames is shown in solid lines, while a similar pulse for data line X.sub.1 for a TFT associated with strobe line Y.sub.3 is shown in broken lines. FIG. 2C shows the resulting voltage applied across the liquid crystal throughout each line frame. The reason why three voltage levels are applied to the X-axis data lines is that it is necessary to apply an alternating current to the liquid crystal to avoid electrochemical degradation as well as to prevent saturation of optical properties. The AC signal results in the RMS (root-mean-square) voltage applied to the liquid crystal being equal to zero over time.
It has been found that the placement of a DC current blocking layer adjacent the pixel electrodes blocks the DC component of the applied voltage and thus permits two voltage level drivers to be used on the data lines. See the above-incorporated U.S. Pat. No. 4,783,146 at column 4, lines 37-40. This may be more clearly understood by reference to FIG. 3, which is the electrical analog of the blocking layer/liquid crystal system. In these systems, the resistivity of the blocking layer is much higher than the resistivity of the liquid crystal layer R.sub.1. Therefore, the resistivity of the blocking layer may be neglected. C.sub.1 and C.sub.2 represent the capacitance of the liquid crystal and blocking layers, respectively. As a result, the application of a signal to the liquid crystal which contains an average DC component, causes the blocking layer to charge to the average value of the applied wave, thereby automatically removing the DC component of the liquid crystal. It is therefore possible to apply an unbalanced voltage to the elements of the display without exposing the liquid crystal material to a DC current. This could allow the reduction in the cost of drivers and complexity of circuitry by eliminating the necessity of having three voltage-level drivers.
However, in attempting to eliminate the three-voltage-level drivers, another problem occurs. Assume that the voltage waveform shown in FIG. 4A is applied to the liquid crystal blocking layer. The resultant waveform across the liquid crystal is shown in FIG. 4B, where it is assumed that there is sufficient current leakage either in the liquid crystal or the TFT providing the applied voltage to discharge the liquid crystal in about one cycle or one frame time (i.e., before another data signal--for the next line of information--is supplied to the pixel electrodes in an image bar or display). However, if the leakage is less (for example, driving with better TFT's or a liquid crystal material that conducts less) and, as a consequence, the liquid crystal layer discharges only partly in one frame time, a problem arises. The resultant AC waveform applied to the liquid crystal blocking layer is shown in FIG. 5A and the voltage to the liquid crystal is shown in FIG. 5B. Since the liquid crystal responds to the RMS (root-mean-square) voltage applied to it, the reduction of leakage has had a profound effect in reducing the RMS on the liquid crystal and thereby also reducing the contrast achievable in the display. Thus, reducing the leakage (an achievement which is otherwise desirable because it allows for more precise control and less deterioration of the liquid crystal) causes decreased performance in the display. Although it is possible to require controlled leakage to prevent this loss of RMS, this solution is unsatisfactory because it requires the close control of leakage parameters in the device, a difficult task considering the difficulty of making TFT's and selecting liquid crystal material with specified amounts of leakage.
As discussed above, ferroelectric liquid crystal materials which are bistable are also used to form light shutters. Ferroelectric liquid crystals have exceptionally high speed as compared with other liquid crystal devices. Because of the bistability and the fact that no direct current can be allowed to flow through the liquid crystal cell without deteriorating it, Lagerwall et al (see Lagerwall et al, 1985 International Display Research Conference, p. 213) has proposed a system in which five different voltages are needed to be applied to the liquid crystal during one access or dwell time (one frame), and drivers capable of delivering four different voltage levels are needed. The difficulty with this procedure is that the time for access to align the ferroelectric liquid crystal is reduced by the necessity to apply certain pulses that are only needed to average the voltage across each pixel to zero. Because of the bistability of the ferroelectric liquid crystal, it is impossible to average the direct current component over more than one frame (i.e., to apply an AC waveform to average the DC component) because of the bipolar nature of the liquid crystal. For example, if one were to apply reverse voltages on alternate cycles, the display would have alternately positive and negative images which would result in a display with little or no contrast in the image presented. An ordinary liquid crystal would only have positive images despite the reverse polarity since ordinary liquid crystals are not bistable and are polarity insensitive.
U.S. Pat. No. 4,783,146 to Stephany et al discloses the use of DC current blocking layers in nematic liquid crystal devices. The use of DC blocking layers permits two-level drivers to be used. See col. 4, lines 37-40. Timing signal waveforms are shown in FIGS. 5 and 8.
U.S. Pat. No. 4,595,259 to Perregaux shows nematic liquid crystal devices used in an image bar.
EP 282,300A discloses a nematic liquid crystal device used as a color display.
A paper entitled "Elasticity and Order of Nematic Liquid Crystals" by Frans Leenhouts, published Dec. 14, 1979 appears to disclose the use of a polymer film of p-xylylene as an alignment layer in nematic-type liquid crystal devices.
U.S. Pat. No. 4,779,958 to Kato et al discloses shielding light sensitive TFT's and extending storage time by providing an opaque overlayer insulated from the TFT's, extending this overlayer and insulator, and combining the extension with an opposing conductor to form a storage capacitor.
U.S. Pat. No. 4,738,515 to Okada et al discloses a driving method for a ferroelectric liquid crystal device wherein a dielectric layer (which is also an alignment layer) is formed over the electrodes of the liquid crystal to prevent shorting. This layer also prevents a reversal phenomenon. See col. 7, lines 1-44.
Other patents disclosing the use of capacitance layers in liquid crystal devices include U.S. Pat. No. 4,728,172 to Cannella, U.S. Pat. No. 4,728,174 to Grinberg et al, U.S. Pat. No. 4,728,175 to Baron, and U.S. Pat. No. 4,840,460 to Bernot et al.
U.S. Pat. No. 4,759,610 to Yanagisawa discloses a liquid crystal device having a light shielding layer which covers the TFT's.
U.S. Pat. No. 4,212,010 to Walter discloses control of a bistable liquid crystal display.
U.S. Pat. No. 4,295,137 to Haugsjaa discloses an AC address signal whose voltage averages to zero.
U.S. Pat. Nos. 4,386,352 and 4,586,039 both to Nonomura et al, disclose the use of waveforms which must average zero DC.
As further background regarding methods of driving liquid crystals and ferroelectric liquid crystals, see U.S. Pat. No. Re. 33,120 and U.S. Pat. No. 4,769,659. All patents cited in this specification are herein incorporated by reference.